1. Field Of The Invention
The field of the present invention relates to high voltage, switched mode power supplies. More particularly, the field of the present invention relates to a high voltage power supply incorporating a plurality of phase controlled series resonant half bridge inverters for minimizing harmonic distortion and for maximizing high voltage and current output.
2. Description of the Related Art
High power lasers such as those used in atomic vapor laser isotope separation (AVLIS) require power supplies capable of delivering substantial voltage (.about.700 volts) at substantial current (.about.200 amps), or on the order of 150 kW. Ideally, such power supplies would have fixed voltage output which does not vary under load and is free of ripple.
Conventional power supplies often are inadequate to the task. First, such power supplies are often operated from three phase current source and therefore have significant amounts of 360 Hz ripple on their outputs.
Second, conventional power supplies generally use thyristors operating at line frequency, or about 60 Hz. Thyristors are used to generate pulses of current into a large transformer that operates conventionally at low frequency, for example 60 Hz. Thyristors switch off and on to regulate the rectified voltage output on the secondary winding of the transformer. This distorts the line current feeding the thyristor regulator circuit by inducing current pulses or surges. Thus conventional power supplies have the disadvantage of large surges in current on the lines to the thyristor regulator circuit. These current surges cause increased line losses and resistive losses in the lines feeding the power control circuit.
Third, thyristors in conventional power supplies are conducting for only a portion of each half sine wave of the line voltage. The current only flows for a portion of the input half sine wave. That is, current only flows for a portion of the time, when output power is fixed. The input current must go to higher peak values to maintain the output power. This, again, causes increased power losses due to current; distortions on the input lines.
Fourth, in those conventional power supplies which have attempted to overcome current distortion on input lines by eliminating the use of thyristors and instead using a large vacuum tube rectifier and/or a vacuum tube tetrode on the secondary side as a series pass regulator, the vacuum tube tetrode acts as a variable resistance in series with a load to maintain a constant voltage across the load. Because the control element is in series with the load being driven, losses occur which produce inefficiency in the system. The control element dissipates power because a voltage develops across the control element while current is flowing through it. Such a conventional power supply causes a loss inside the vacuum tetrode itself and results in low efficiency. The power ratio, or efficiency, of such a power supply would only be around 50-60%.
In an effort to gain efficiency, many switched mode power supplies eliminate the series pass element (tetrode regulator). Conventional switched mode power supplies also may eliminate the large transformer operating at 60 Hz. This is done by using FETs or other switches and switching the FETs on and off at high frequency to develop a high frequency ac voltage on the primary of a smaller transformer. The ac voltage on the primary is intended to produce a larger voltage on the secondary side which can be rectified to produce a high voltage dc output.
There is a need, therefore, for an improved solid state high power switched mode power supply having improved efficiency, reduced power losses and which is capable of supplying a constant highly regulated output. There is also a need in such a power supply for an improved heat dissipation system.
FETs operate at high frequencies and need only a small voltage and a current of approximately 100 microamps to turn on. However, switched mode power supplies incorporating FETs are not practical for high power applications such as in an AVLIS process because of the low power ratings of commercially available FETs.
The FETs also dissipate significant amounts of power in the conducting state. When the currents are high, the resistance of the FETs from drain to source causes a significant voltage drop from drain to source. This voltage drop may represent a significant dissipation of power.
In conventional switched mode power supplies, problems also arise due to increased harmonic distortions of the input current waveform at line frequency. Harmonic distortions of the current waveform at line frequency cause increased power dissipation in transmission lines feeding the power supply input. Switching frequency harmonics are also radiated as noise.
Conventional switched mode power supplies also have problems in dissipating heat which builds up in many of the components. For example in a laser isotope separation process, some components, for example diodes, carry 50 kw of power at 700 volts, or approximately 70 amps. Conventional switched mode power supplies lack efficient heat dissipation means for providing adequate cooling of these components. This results in a decrease in reliability and a shorter useful life for the power supply.
Conductors are frequently expected to carry hundreds of amperes RMS without inordinate heat buildup. Conventional power supplies frequently lack sufficient heat dissipating means to prevent damage in such high power applications.